Q1. (a) Define the term activation energy for a chemical reaction. (b) Draw, with labelled axes, a curve to represent the Maxwell Boltzmann distribution of molecular energies in a gas. Label this curve T 1. On the same axes, draw a second curve to represent the same sample of gas at a lower temperature. Label this curve T 2. Use these curves to explain why a small decrease in temperature can lead to a large decrease in the rate of a reaction. (8) (c) Give one reason why most collisions between gas-phase reactants do not lead to a reaction. State and explain two ways of speeding up a gas-phase reaction other than by changing the temperature. (5) (Total 15 marks) Q2. Gas G decomposes as shown in the equation below. G(g) X(g) + Y(g) (a) Draw, on the axes below, a Maxwell Boltzmann distribution curve for a sample of G in which only a small proportion of molecules has energy greater than the activation energy, E a. (3) Page 2
(b) Define the term activation energy. (c) At any time, most of the molecules of G have energy less than the activation energy. Suggest why, at a constant temperature, most of G eventually decomposes. (d) State the effect, if any, of adding a catalyst on the time required for G to decompose, compared with a similar sample without a catalyst. Explain in general terms how the catalyst has this effect. Time for decomposition... Explanation... (3) (Total 10 marks) Q3. The diagram below represents a Maxwell Boltzmann distribution curve for the particles in a sample of a gas at a given temperature. The questions below refer to this sample of particles. Page 3
(a) Label the axes on the diagram. (b) On the diagram draw a curve to show the distribution for this sample at a lower temperature. (c) In order for two particles to react they must collide. Explain why most collisions do not result in a reaction. (1) (d) State one way in which the collision frequency between particles in a gas can be increased without changing the temperature. (1) (e) Suggest why a small increase in temperature can lead to a large increase in the reaction rate between colliding particles. (f) Explain in general terms how a catalyst works. Page 4
(Total 10 marks) Q4. The total area under the distribution curve represents energy A B C D total energy. activation energy. total number of reacting molecules. total number of molecules present. (Total 1 mark) Q5. (a) State what is meant by the term activation energy of a reaction. (1) (b) State in general terms how a catalyst increases the rate of a chemical reaction. Page 5
(c) The curve below shows the Maxwell Boltzmann distribution of molecular energies, at a constant temperature, in a gas at the start of a reaction. On this diagram the most probable molecular energy at this temperature is indicated by the symbol E mp and the activation energy by the symbol E a. Consider the following changes. (i) The number of molecules is increased at constant temperature. (ii) (iii) The temperature is decreased without changing the number of molecules. A catalyst is introduced without changing the temperature or the number of molecules. For each of these changes state how, if at all, the following would vary: the value of the most probable energy, E mp the number of molecules with the most probable energy, E mp the area under the molecular energy distribution curve the number of molecules with energy greater than the activation energy, E a (12) (Total 15 marks) Q6. (a) Below is a Maxwell Boltzmann curve showing the distribution of molecular energies for a sample of gas at a temperature T. Page 6
(i) Label the axes on the diagram above. (ii) What does the area under the curve represent? (iii) State why this curve starts at the origin. (4) (b) (i) State what is meant by the term activation energy. (ii) The rate of a chemical reaction may be increased by an increase in reactant concentration, by an increase in temperature and by the addition of a catalyst. State which, if any, of these changes involves a different activation energy. Explain your answer. Change(s)... Explanation... Page 7
(5) (Total 9 marks) Page 8